The front of the single pipe is a terminating block under the transmission tunnel, the single pipe splits into 2 pipes within this block, the 2 pipes then run forward to the usual 2 pipe AC stuff under the bonnet. I haven't had a look at the rear evaporator yet, it may be that there's some sort of single pipe to 2 pipe adaptor before the evaporator unit or it may be that the evaporator unit is connected directly to the single pipe.
Might be possible to remove the single pipe and just weld the affected area... but the last time I tried aluminium welding myself was using a mig on a Vauxhall Senator's heater pipe and it didn't go so well! If I were to do it again I'd practice first on a bit of AC pipe removed from a scrap vehicle. Wonder if they are aluminium or an aluminium alloy though.